Valve-regulated lead-acid (VRLA) batteries rely upon internal gas recombination to minimize electrolyte loss over the life of the battery, thereby eliminating the need for re-watering. Internal gas recombination is achieved by allowing oxygen generated at the positive electrode to diffuse to the negative electrode, where it recombines to form water and also suppresses the evolution of hydrogen. The diffusion of oxygen is facilitated by providing a matrix that has electrolyte-free pathways. The recombination process is further enhanced by sealing the cell with a mechanical valve to keep the oxygen from escaping so it has greater opportunity for recombination. The valve is designed to regulate the pressure of the cell at a predetermined level, hence the term, "valve-regulated". It should be noted that the term "sealed" is often times used in the art when referring to VRLA batteries. This usage is somewhat incorrect as some type of means for venting a battery cell must be provided to vent gasses generated during charging.
VRLA recombination batteries offer a number of advantages compared to flooded cell batteries. For example, the very low levels of gas discharge from this type battery permit its use for office equipment. Also, because the electrolyte is held in a matrix, there should be no electrolyte splash or spillage if the cell case is cracked or otherwise damaged. The cell has the potential to operate for some time with a cracked case. VRLA batteries are particularly suited for remote back-up power applications because they do not require the same type of periodic maintenance required by flooded cells. These differing maintenance requirements provide cost savings when VRLA batteries are used.
The liquid electrolyte in VRLA batteries is absorbed in plate separators which operate in a nearly completely saturated state. Desirably, a saturation level of between about 90 and about 99 percent is maintained in the separators. Saturation level is defined as the ratio of the weight of the electrolyte actually absorbed in the separators to the weight of the acid required to completely saturate the separators. Other definitions of this term and of the concept of saturation level are known in the art. As described above, it is necessary that some air space remain in the separators for proper recombination.
Although the separators will operate in a less than totally saturated condition when the VRLA cell is placed in service, in some instances the VRLA cell container is flooded with electrolyte when it is "formed". The term "formed" as used in the battery manufacturing art refers to placing the initial charge on the individual battery cells by passing a forming current therethrough. The present invention is concerned with the manufacturing steps that take place after a VRLA cell has been formed while in a flooded state. This temporary condition of being flooded should be distinguished from flooded lead acid cells which by design operate with plates that are submerged in electrolyte. Thus, any reference herein to a "flooded" cell refers to a VRLA cell soon after it has been formed and not to a cell intended for normal operation in a flooded state.
Excess electrolyte must be removed from VRLA batteries, after forming, so as to reach the desired level of saturation. Significantly, the amount of electrolyte remaining in the separator material must be controlled carefully for optimum battery performance. The separator material must not be completely saturated with liquid but must be left at something less than complete liquid saturation so that gas passages are provided. These gas passages are needed to facilitate the recombination process described above.
Several manufacturing techniques have been employed to remove the excess acid from the flooded VRLA cell. One method is to simply dump the excess acid from the cell relying on the weight of the cell after dumping to determine that the proper saturation level has been attained. This method is not desirable because it is messy and raises dangers from splashing corrosive electrolyte material. Moreover, it is difficult to accurately control the final saturation level of the AGM material using this method. Typically, cells so created initially are somewhat too wet to recombine with high efficiencies and must be allowed time to dry out to provide highly efficient recombinant batteries.
Another technique uses tank formed plates. The cells are then assembled using these plates and the subsequent acid fill is controlled to provide a specific unsaturated condition. While this process can provide a consistently reproducible product, it is tedious, time consuming and is one of the most expensive options.
Still another technique involves adding a controlled amount of liquid electrolyte to the unformed cell. One process for doing so is disclosed in U.S. Pat. No. 5,731,099 to Badger et al. This patent discloses an apparatus for introducing a controlled volume of an electrolyte to a battery case. Various other methods employ some aspects of these and other approaches. Not all of these approaches are discussed here.
There is a need then for a method of removing electrolyte from a "flooded" VRLA battery cell during the process of making a VRLA battery. The present invention addresses this and other needs. Further, the present invention provides additional advantages and solutions to additional problems not necessarily stated herein. The scope of the present invention includes those advantages and solutions to these additional problems.